Researchers and engineers have been intensely developing the following two types of superconducting wire as the one using an oxide superconducting material. One is a tape-shaped silver-sheathed superconducting wire that is produced by the power-in-tube method and that is mainly composed of a (Bi, Pb)2Sr2Ca2Cu3O10±δ (“δ” is a number of 0.1 or so, and hereinafter referred to as (Bi, Pb)2223) phase (see, for example, Nonpatent literature 1). The other one is a tape-shaped thin-film superconducting wire produced by forming a superconducting layer on a metallic substrate by using the gas phase method or the liquid phase method. The superconducting material in the thin-film superconducting wire is an oxide superconducting material expressed as the chemical formula RE1Ba2Cu3Ox (“x” is a number close to 7, and hereinafter referred to as RE123). In the above formula, RE (rare earth) represents one of the rare-earth elements, such as Y, Ho, Nd, Sm, Dy, Eu, La, and Tm, or a mixture of them. (See, for example, Nonpatent literature 2.)
A superconducting coil to be used in a magnetic field has been produced by using the above-described superconducting wire. Patent literature 1 has disclosed a superconducting coil formed by stacking in layers a plurality of pancake coils that use a tape-shaped (Bi, Pb)2223-based superconducting wire. The superconducting coil produced by using the tape-shaped (Bi, Pb)2223-based superconducting wire is cooled to a temperature as low as 20 K or below to carry an intended operating current to generate a magnetic field.
The (Bi, Pb)2223-based superconducting wire has not sufficient resistance to a magnetic field. Consequently, when a magnetic field is applied, its critical-current value decreases significantly. A coil formed with the wire decreases its critical-current value even by the magnetic field generated by the coil itself. As a result, the operating temperature is reduced to increase the critical-current value so that a sufficient superconducting current can be fed into the coil even in the generated magnetic field. As described above, when a comparatively high magnetic field is intended to generate in a superconducting coil composed of the tape-shaped (Bi, Pb)2223-based superconducting wire, it is necessary to cool the superconducting coil to a temperature as low as 20 K or so.
On the other hand, the tape-shaped thin-film RE123-based superconducting wire has a higher resistance to a magnetic field than that of the (Bi, Pb)2223-based superconducting wire. Consequently, it has a high critical-current value even at a relatively high temperature in a magnetic field. This feature enables the formation of a superconducting coil that generates a high magnetic field even at high temperatures. In addition, in a current-voltage characteristic in a superconducting state expressed as V ∝ In (V: voltage, I: current), the RE123-based wire has a high n-value, which is the exponential number to the current. In other words, it is a wire in which the voltage varies sensitively to the variation in the current. When a superconducting coil is formed, the high n-value becomes not only advantageous but also disadvantageous.
When a superconducting coil composed of a wire having a high n-value is operated at a current not more than its critical current, the generated voltage is extremely low and therefore the heat generated in the superconducting coil is small. On the other hand, when the operating current exceeds the critical-current value due to, for example, the temperature rise resulting from the entering of an external disturbance for some reason, a high voltage is generated and consequently a large amount of heat is generated. This heat generation causes the quenching phenomenon in the superconducting coil. When the quenching phenomenon occurs excessively, the superconducting wire will be burnt out and the superconducting coil may be broken.    Patent literature 1: the Japanese patent application Tokuganhei 10-104911 (the published Japanese patent application Tokukaihei 11-186025)    Nonpatent literature 1: SEI Technical Review (SEI: Sumitomo Electric Industries), July 2006, No. 169, pp. 103 to 108    Nonpatent literature 2: SEI Technical Review, July 2006, No. 169, pp. 109 to 112.